1. Field of the Invention
The invention relates to a substrate for a light-emitting element based on an organic light-emitting material, a method of manufacturing the same, an electrode for a light-emitting element, and a light-emitting element having the same.
2. Description of the Background
A transparent substrate is generally used as an electrode in an organic light emitting diode (OLED) display or in a light-emitting element. In an OLED display, a layer formed of indium tin oxide (ITO) that is disposed on a base substrate is generally used as an anode.
In order to operate efficiently, OLED elements may have a variety of properties such as electron conductivity, hole conductivity, and light emission, but most materials that are used for an OLED satisfy only one of these properties. Multilayered elements in which different layers are combined may be used to increase efficiency. For example, one layer may have excellent hole conductivity and another layer may have a better electron conductivity.
In OLED applications, a hole injection layer (HIL) may be used on an ITO layer to increase the efficiency of an anode comprising a base substrate and an ITO layer. Polyethylene dioxythiophene/polystyrene sulfonate (PEDT/PSS) may be used as an HIL. One problem with this HIL is that the surface of the ITO layer becomes etched due to the acidity of the PEDT/PSS and ion diffusion and penetration to an organic layer of an OLED cannot be completely avoided. The ions adversely affect the durable lifespan of the OLED elements (Nucl. Inst. and Meth. In Physics Res. B194 (2002) 346; Appl. Phys. Lett. 75 (1999) 1404; Appl. Phys. Lett. 81/6 (2002) 1119; Mat. Sci. Engin. B97 (2003) 1-4; J. Appl. Phys. 79 (1996) 2745).
In order to maintain the same efficiency and increase the durable lifespan, it is desirable to use a substrate that has an electrode material other than ITO that is more resistant to PEDT/PSS. The electrode material is preferably inexpensive, translucent, and has high electric conductivity. In addition, the electrode material may be flexible so that it can be applied to flexible elements such as organic display elements or organic solar cells.
Polyethylene dioxythiophene (PEDT) with high conductivity is known as a substitute for ITO. Highly conductive PEDT such as that produced by Bayer (“in situ” PEDT with conductivity of 500 S/cm) has a very large voltage loss with large surface OLED elements when it is used as an ITO substitute (the conductivity of ITO is 104 S/cm). As such, the brightness of the OLED elements degrades as the distance from voltage contacts increases.
WO 03/106571 A1 discloses increasing the conductivity of PEDT/PSS by replacing a solvent. PEDT/PSS is typically water-soluble and has a conductivity of up to 10−3 S/cm (H. C. Starck Baytron P TP A1 4083). Through the formulation of a soluble polymer, PEDT can have a conductivity of up to 130 S/cm (H. C. Starck Baytron F CPP 105D M) or 120 S/cm (Agfa Orgacon foil) or 500 S/cm (H. C. Starck polyster foil JOF 6073 coated with “in situ” PEDT).
By replacing water with an alcohol such as ethylene glycol, the conductivity of a PEDT/PSS solvent may be increased by a factor of two. According to WO 03/106571 A1, a PEDT/PSS solvent with a conductivity of up to 10−1 S/cm can be achieved. Thus, the conductivity of the PEDT/PSS solvent is not sufficiently increased with the replacement of ITO as an anode material for organic elements. It is also disadvantageous that the stability of a PEDT/PSS alcohol solvent is low. Due to agglomeration and coagulation that occurs after a predetermined amount of time, printability or homogenous processing during spin coating becomes difficult and the durability of the PEDT/PSS solvent diminishes.